The present invention relates to heating, ventilation and air-conditioning (HVAC) systems. More particularly, the present invention relates to variable air volume HVAC systems.
Increased emphasis is being placed in the quality of air within occupied buildings, and therefore increased emphasis is being placed on introducing the correct amount of outdoor air into those buildings. The trend toward xe2x80x9ctighterxe2x80x9d buildings has resulted in less outdoor air infiltrating into buildings, making it more important that the mechanical ventilation systems introduce the specified amount of outdoor air.
The task of consistently introducing the specified amount of outdoor air into a building is complicated by the fact that many mechanical ventilation systems are the variable air volume type (VAV). On VAV systems, the air delivery volume drawn in by the fans changes. Fan speed is varied and therefore the negative pressure those fans create at the inlet to the outdoor air intake damper also changes. The amount of outdoor air that will be drawn in through the outdoor air damper is dependent on two things: how far open the damper is, and the negative pressure generated by the fans at the damper inlet. If a consistent volume of outdoor air is to be drawn into the building, the damper open position must change whenever the negative pressure at that damper intake changes.
A proposed draft of ASHRAE Standard 62-1989R has included Section 5.6.9.1 to deal with the problem of bringing in the specified cubic feet per minute (CFM) of outdoor air with a VAV system. That section stated, xe2x80x9cVariable air volume systems (except those supplying 100 percent outdoor air) shall include controls and devices to measure outdoor air mass flow at the air handler and designed to maintain outdoor air mass flow not less than 90 percent of required level over the expected supply air operating range.xe2x80x9d Although this revised standard is still in the proposal stage, the requirement for direct measurement of outdoor air is showing up in project specifications. Accordingly, there is a present need in the industry for an accurate air mass flow measuring device, especially being accurate at relatively low air mass flow rates.
Products are now on the market that attempt to measure and control outdoor air CFM using a calculation based on a measurement of velocity, velocity pressure, or louver pressure drop static pressure. Outdoor air mass flow volume is calculated by using that measured air velocity or velocity pressure and an equivalent duct area, or by using the measured static pressure drop across the outdoor air intake louver and typical louver pressure drop characteristics. In reality, the velocity pressures or static pressures encountered at the outdoor air intake are so low at the minimum outdoor air mass flows that need to be measured, that it is not reasonable to use them for what is supposed to be an accurate measurement. To accurately measure air velocities, ideal conditions must exist, such as long, straight duct runs and uniform air velocities throughout that.duct and known air densities. The outdoor air intake on a typical roof-mounted air handling unit will have a tortuous, turbulent outdoor air mass flow path, widely varying temperatures, changing barometric pressures, and varying wind conditions, which cumulatively make it unsuitable for the aforementioned existing type of measurement techniques. It becomes increasingly difficult to accurately measure air mass flow rates as that air mass flow rate is reduced. The purpose of measuring the air mass flow rate is to be sure that the flow rate does not get below the specified minimum at that low end of its range, typically 10 to 20 percent of the maximum air mass flow rate.
CONTROLLING FLOW OF OUTDOOR AIR: To deliver the specified volume of outdoor air to the building, the present invention measures outdoor air CFM flow rate, and controls the position of the outdoor air damper to maintain the specified CFM flow rate.
The present invention is a sensing vane that is rotatably displaced by the impingement and flow of an air stream across it. That vane repeatedly and accurately assumes a position according to the mass of air flowing across it. The xe2x80x9cvane positionsxe2x80x9d are translated into air mass flow readings of xe2x80x9cstandard airxe2x80x9d (0.075 lbs./cu. ft.). Therefore, vane position readings always indicate xe2x80x9cstandard airxe2x80x9d because the vane is responding to air mass flow that is, air weight (mass) rather than air volume.
On this illustrative version of the device, the air mass flow that causes displacement of the vane is opposed by a combination of two springs and gravity. Alternative models can use other combinations of springs and gravity, or only gravity, or only springs.
There are many ways to translate the vane position to an air mass flow reading. A simple way would be to have the vane align with a suitable marked scale and directly read air mass flow. Another way is to have the vane position control an electrical signal such that the electrical signal can be translated into an air mass flow reading. Connecting the vane to a potentiometer is one way to accomplish that result.
Through tests we have determined that the displacement of the vane of the present invention accurately and repeatedly indicates the air mass flow.
SENSING VANE: The sensing vane of the present invention functions according to the principals described here but it is adapted to meet the requirements for specific applications. The application will influence the vane size, location, and orientation. The vane adaptation in this illustration is tall and narrow, with a vertical pivot axis Alternatively, a vane could be long and narrow on a horizontal axis to be compatible with horizontal ductwork or arranged for vertical air mass flow.
It is important that the blade rotational friction be minimized. It must be free-swinging to respond to small forces. In this case, virtually all of the weight of the vane is on the lower hinge. The lower hinge or pivot is essentially a conical recess that rests atop and pivots on a fixed, sharp point in order to minimize rotational friction. The upper bearing is a nylon or other minimal friction bushing that keeps the pivot axis in alignment but has little static force on it.
This sensing vane does not add measurable pressure drop to the outside air intake path. At minimum air mass flow, the vane is somewhat perpendicular to air mass flow, but because of the low flow rate there is not a measurable pressure drop. As air mass flow increases, the vane rotates, becoming increasingly more parallel with the air mass flow path and eventually reaching a position where it has swung parallel with and proximate the backwall of the outdoor air intake, essentially out of the air path.
The vane is mounted to a vertical support that is attached to the backwall of the outdoor air intake. That vertical support includes a leading edge lip that overlaps and protects the leading edge of the vane from air mass flow impingement. That lip prevents a turbulent, high-velocity air stream from getting behind the vane (between the vane and the air intake backwall) and causing the vane to flutter.
SPRINGS: Optimum accuracy will result if the Vane Position vs CFM relationship is characterized such that similar changes in CFM will result in similar changes in the vane position, indicating a linear relationship. Generally, very light air mass flow forces must move the vane near the minimum air mass flow position, building up to heavy force near mid-rotation, and then dropping back as the maximum air mass flow position is approached. On spring versions of the present invention, the desired linearity is accomplished by the selection and levering of the springs to act against the forces at various points of vane rotation.
The exemplary embodiment uses springs to oppose the movement of the vane. Two extension springs oppose the force of the air mass flow against the vane. The springs and link arms are such that, at very low air mass flows, only a light spring opposes vane displacement. As the air mass flow increases, the force on the vane increases and, correspondingly, a second, heavier spring engages. As the air mass flow increases further, the leverage working with that heavier spring changes, reducing the rate of increase with which the heavier spring opposes further vane movement. This is necessary because, as the vane becomes less perpendicular to air mass flow (more nearly parallel to the intake backwall), it takes less proportional spring force to oppose the air mass flow induced vane displacement.
Extension springs normally have two ratings: the spring xe2x80x9cRatexe2x80x9d and the xe2x80x9cInitial Tension.xe2x80x9d Coil extension springs are normally wound with adjacent coils in contact with each other when in the relaxed condition. The force to separate the coils is the Initial Tension. The application of springs for the device of the present invention requires precision springs. Spring manufacturers have a difficult time maintaining an accurate and consistent Initial Tension. Therefore, we have these springs xe2x80x9cOpen Woundxe2x80x9d so there is no coil contact when the spring is relaxed and accordingly, there is no Initial Tension. Further, the required accuracy for the spring Rate is specified.
SENSOR POTENTIOMETER: A rotary potentiometer may be advantageously used as the vane position sensor in the exemplary device. The potentiometer shaft can be direct-coupled to the vane shaft. The potentiometer is then essentially mounted by its shaft to the vane. A thin, flexible bracket is used to restrain the body of the potentiometer from rotating, but allow the sensor to free-float in any direction, thus the potentiometer is self-aligning with the vane and free from binding forces.
CALIBRATION: A preferred embodiment uses a vane that is primarily influenced by pre-adjusted precision springs, except at minimum air mass flows. Accurate low air mass flow measurement results require that, after installation, the vane assembly be accurately leveled so the gravity effect on the vane will be correct. This may be accomplished by using a miniature cable and a weight. The miniature cable is horizontal, with one end attached perpendicular to and near the outer edge of the vane. The other end of the cable is anchored to the backwall of the intake cabinet. When a calibration weight is hung at the center of the cable, it places a known force on the vane, a force on the blade that is equivalent to the force caused by a specific air mass flow. With that weight in place, the vane assembly is leveled, being adjusted to a position that will result in the appropriate CFM reading from the sensor for the known calibration weight.
In an alternative embodiment, calibration is accomplished by using a weight and lever device. The weight causes a known force to be applied against the vane and at a specific location. That force on the vane at that location is equivalent to the force caused by a specific air mass flow when the unit is correctly leveled. With the weight and lever device in place, the vane assembly is leveled, being adjusted to a position that will result in the appropriate CFM reading from the sensor for the known calibration weight.
The present invention is a flow measuring device for use with a heating, ventilation and air-conditioning (HVAC) system and includes a vane being positionable in an airstream, the vane being rotatably positionable between a first minimum air mass flow disposition and a second maximum air mass flow disposition, the vane being biased in the first minimum air mass flow disposition, the disposition of the vane being responsive in part to an impingement of an air mass flow on the vane. A measuring device is operably coupled to the vane for measuring the disposition of the vane and for providing an output communication of the vane disposition, the vane disposition having a known relationship to the air mass flow. A method of use of the flow measuring device and calibration of the flow measuring device are included in the present invention.